Method for dry electrostatic coating

ABSTRACT

A METHOD FOR DRY ELECTROSTATIC COATING OF BASE METAL SURFACES TO PROVIDE CONTINUOUS, EVEN, RESIN FILMS FREE OF PIN-HOLES AND CISSINGS WITHOUT THE NECESSITY OF PRE-HEATING THE BASE METAL BY COATING THE SURFACE OF THE BASE METAL WITH A PRIMER COMPRISING A VEHICLE AND AN ELECTROCONDUCTIVE PARTICULATE MATERIAL HAVING A PARTICLE SIZE OF NOT MORE THAN 100U, SUCH AS ALUMINUM POWDER, IN AN AMOUNT NECESSARY TO PRODUCE A DRY FILM HAVING A VOLUME RESISTIVITY OF 10**9 TO 10**14 OHM-CM., AND DRYING THE FILM.

United States Patent Oflice US. Cl. 117-17 Claims ABSTRACT OF THE DISCLOSURE A method for dry electrostatic coating of base metal surfaces to provide continuous, even, resin films free of pin-holes and cissings without the necessity of pre-heating the base metal by coating the surface of the base metal with a primer comprising a vehicle and an electroconductive particulate material having a particle size of not more than 100g, such as aluminum powder, in an amount necessary to produce a dry film having a volume resistivity of to 10 ohm-cm, and drying the film.

This application is a division of application Ser. No. 824,280 in the name of Taizo Kondo et al., filed May 13, 1969 (now abandoned).

This invention relates to an improved primer for electrostatic spraying of resin powder and a process for dry electrostatic coating of base metals with synthetic resin powder.

In recent years dry electrostatic coating method is widely being utilized for coating the surface of metal products with resins in various fields, since this process provides a number of important advantages as compared with liquid electrostatic coating methods. According to the principle of the conventional dry electrostatic coating methods a negative electrostatic charge is put on resin particles and the charged particles are sprayed to be brought near the base metal which is electrically grounded. Thus the particles are attracted to the base metal and deposited on it by Coulomb energy without loosing their charge due to nonconductivity thereof. The deposited particles are then fused by heating in an oven to produce a continuous coating film.

According to the conventional methods, however, when the deposited particles are fused on the base metal they will lose Coulomb energy by the discharge due to rapid drop of electric resistance in proportion to the temperature, resulting in the particles easily falling off the surface of the base metal. The higher the melting point of the resin particles the more they are subject to this trend. When polyamide particles having a melting point of higher than 150 C. are directly deposited on a base metal by dry electrostatic coating method and fused on the base metal, for example, they discharge before they are heated to their melting point at which they coalesce with one another, leading to the deposited particles falling off. For this reason it has been considered difiicult to produce a continuous polyamide coating film on a base metal by dry electrostatic coating methods. Even when polyvinyl chloride particles are deposited on a base metal they are liable to fall off in the melting step unless rapidly heated to their melting point.

To overcome this drawback and improve the properties of the resin coatings it is known in the art that primers may be applied to the surface of base metals prior to dry electrostatic coatings. If a primer is used, however, the electric resistance on the surface of the base metal coated therewith becomes so high that the base metal surface will easily have the same charge by the deposition of extremely small amounts of resin particles to cause the mutual repulsion of the particles on the surface, preventing deposition in a desired thickness. Thus the resultant molten film on the base metal is generally thin and finished withnumerous pin holes or cissings and also with uneven thickness.

Various attempts have been made to solve these problems, but no successful methods have been proposed yet. In one known method, for example, a base metal is preheated to a temperature higher than the melting point of the resin particles deposited on the surface thereof so that the particles deposited are instantly melted on the surface to produce a continuous coating film. According to this method, however, a device for heating the base metal to such a high temperature is required, resulting in the loss of the inherent merit of electrostatic coating which requires no preheating of the base metal.

Another countermeasure is to preheat the base metal coated with a primer to about 50 to C. so as to reduce the electric resistance of the particles to be deposited on the surface of the base metal by the subsequent electrostatic coating, and thereby mutual repulsion of the resin powder is minimized, making it possible to obtain the deposition of the particles in a desired thickness. According to this method, however, the release of the charge on the deposited particles reduces Coulomb energy, which results in poor adhesion of the particles to the metal base as easily falling off by relatively weak shock, such as conveyor line vibration. Further such method requires preheating device and complicated procedure for controlling the temperature of the heated base metal.

One object of the invention is accordingly to provide an improved primer for dry electrostatic coating which makes it possible'to deposit resin particles in a desired thickness by dry electrostatic coating.

Another object of the invention is to provide improved primer for dry electrostatic coating which makes it possible to produce by dry electrostatic coating a continuous and even resin film which is firmly adhered to a base metal and has a desired thickness free from pin holes and cissin gs without preheating of the base metal.

A further object of the invention is to provide a method of electrostatic coating of a base metal with resin powder, whereby deposition of resin powder in a desired thicknes may be secured.

A still further object of the invention is to provide -a method of electrostatic coating of a base metal with resin powder, which makes it possible to produce a continuous and even resin film which is firmly adhered to a base metal and has a desired thickness free from pin holes and cissings, without any preheating of the base metal.

These and other objects of the invention will be apparent from the following description.

The primer composition for dry electrostatic coating of the invention'comprises a vehicle and a particulate electro-conductive material dispersed in the vehicle; said electro-conductive material having a particle size of not more than p. and being contained in the amount ranging Patented Aug. 27, 1974 from to 60 weight parts, based on 100 weight parts of the binder resin in the vehicle and necessary for producing a dry film having a volume resistivity of 10 to 10 ohm-cm.

Throughout the specification and claims "volume resistivity of the dry film of the present primer is defined from the following equation.

Volume resistivity (ohm-cm.) X R wherein t is thicknes of dry film (cm.), A is area of main electrode (cm?) and R, is electric resistance measured in accordance with ASTM257-61 in which mercury electrode is used and 100 v.-D.C. is applied using a test piece prepared by spraying a primer of the invention onto the surface of a degreased steel plate, 100 mm. x 100 mm. x 0.8 mm., to a thickness of 20, in terms of dry film, followed by drying, and repeating this procedure to produce a dry film of 500 thick on the steel plate.

The vehicle in the present primer is an organic solvent solution of a binder resin. Representatives of the binder resins are, for example, phenolic resins, vinyl chloride resins and epoxy resins, though other polymers may also be used for the purpose. These resins may be used singly or in mixture with one another or with acrylic resins. Although preferable resins may be selected in accordance with the kinds of the polymer powder to be deposited by electrostatic coating and the properties of the electrostatic coating film required to be improved, usually phenol resin, epoxy resin and polyvinyl chloride may preferably be used singly or in mixture with one another. The solvent used includes various organic solvents capable of dissolving the binder resin used, such as toluene, xylene, mineral spirits, methylethyl ketone, etc. The resin concentration of the solution may vary in accordance with the kinds of the resin used and the methods for coating, but it is preferable to be in the range of to 50 weight percent, based on the combined weight of the resin and organic solvent. The most preferable concentration is 15 to weight percent for polyvinyl chloride, to weight percent for phenol resin and 30 to 40 weight percent for epoxy resin.

The particulate electro-conductive materials used in the invention include powder of, for example, aluminium, silver, copper, bronze, carbon black, acetylene black, graphite, etc., most desirable being aluminium powder and graphite powder. The particle size of the materials is not larger than 100 preferably in the range of 0.01 to 10p. The amount of the electro-conductive material is critical and it should be in the range of 10 to weight parts, based on 100 weight parts of the binder resin in the vehicle. Further, by the addition of the electro-conductive materials Within the above range the volume resistivity of the dry film defined before must be in 10 to 10 ohmcm. Namely, the electro-conductive materials should be added in the amount necessary for producing the dry film of the primer having a volume resistivity of 10 to 10 ohm-cm. within the rtnge of 10 to 16 weight parts, based on 100 weight parts of the binder resin. This amount is far high as compared with that of pigment used in the conventional primer. And unless the amount of the electroconductive materials is within the above range, by dry electrostatic deposition there can not be obtained a continuous and even resin film which is firmly adhered to a base metal and has a desired thickness free from pin holes and cissings. For example, when the conductive materials are added in less than 10 weight parts, based on 100 weight parts of the binder resin, or if the volume resistivity of the dry film of the primer is higher than 10 ohm-cm, such primer results in the production of a thin film having undesired pin holes or cissings. And when the conductive materials are added in more than 60 weight parts, based on 100 weight parts of the binder resin, or if the volume resistivity of the dry film of the primer is lower than 10 ohm-em, the resin particles deposited on the primer film by electrostatic spraying easily fall off in melting step.

The preferable amount of the conductive materials is in the range of 20 to 50 weight parts, based on weight parts of the binder resin, and the preferable volume resistivity of the dry film of the primer is in the range of 10 to 10 ohm-cm.

Though pigments or fillers may be added to the present primer, it is not desirable in the invention. When such additives are required it is preferable to add them in as small amount as possible, so as not to allow the undesired coagulation of the primer.

According to the invention, the surface of a base metal is coated with the present primer by various means, such as dipping, brushing, casting, rolling, spraying, electrostatic spraying, etc. and dried to obtain the dry film of the primer having a thickness of more than 3pc, preferably 10 to 100a.

Then resin powder is deposited on the primer film by the conventional dry electrostatic coating methods. As the primer film has a volume resistivity of 10 to 10 ohm-cm. the resin powder may be deposited to the desired thickness without mutual repulsion. The base metal is then heated in an oven to melt the deposited resin powder, whereby a continuous and even resin film having a desired thickness can be obtained free from undesired pin holes and cissings for the presence of the specific primer film of the invention.

The resin powder used in the electrostatic coating may include any one which has been used in the conventional dry electrostatic coating, for example, epoxy resins, olefin polymers, polyesters, polyamides, vinylchloride polymers, polycarbonates, cellulose derivatives, etc. These resins may be used singly or in mixture with one another. The resin powder may contain pigments, fillers and other additives, such as a plasticizer, curing agent, stabilizer, etc.

For better understanding of the invention Examples are given below, in which all parts and percentage are by weight.

EXAMPLE 1 100 parts of methylisobutylketone solution of resol type phenol resin containing 50 percent nonvolatile material, 100 parts of methylisobutylketone and 50 parts of cyclohexanone were mixed by stirring, to which was added non-leafing aluminium powder passing a 325 mesh screen in the proportions as shown in Table 1, and was further stirred homogeneously for 30 minutes, whereby a primer was obtained.

100 parts of polyamide particles and 5 parts of rutiletype titanium dioxide were mixed by a Henshell mixer at 3000 rpm. for 10 min., whereby the polyamide powder composition for dry electrostatic coating was prepared. The resultant product had the following characteristics:

Grain size All passing mesh. Softening point 150 C. Volume resistivity 10 ohm-cm.

A cold-rolled steel sheet, 200 mm. x 100 mm. x 0.8 mm. which was subjected to zinc phosphate parkerizing in accordance with Bonderite 144 standard process was coated by spraying with the primer of Table 1 below to dry film thickness of 10 and after being left at room temperature for 5 min. was heated in an air oven at i5 C. for 10 min. and then cooled. Then the test piece was coated with the above polyamide powder composition with R-EaP gun Trademark, electrostatic powder spray coating machine, of Ransburg (Japan, Ltd., Japan) at jet velocity of 210 g./ min. for 10 see. with uniform finish, and immediately heated in an air oven at 200i-5 C. for 4 min. holding the coated surface vertical, and then cooled.

In the heating process the softening point of the polyamide powder composition, 150 C., was reached in 135 sec.

TABLE 1 Experiment number 1 2 3 4 5 6 Composition of primer:

Phenol resin 100 100 10 1 100 No primer used. Aluminium powder 0 i 20 60 150 Volume resistivity (ohm-em.) 10 2Xl0 1Xl0" 3X10 1X10 Results of experiment:

Falling 01% during heating powder N n None None N n Complete lalllng.... Completefalling.

compos 1on. Average thickness of molten film 4).. 139 140 270 260 Uniformity of molten film Cissing Cissmg... Good".-- Good As evident from the results of Table 1, when the volume resistivity of the dry film of the primer was less than ohm-cm., or where no primer was used and the base metal was coated with polyamide powder composition direct by electrostatic spray coating, the polyamide powder composition deposited on the vertical surface started falling during the heating step and failed to form a film, whereas when it was more than 10 ohm-cm. the amount of deposited powder decreased throughout, and

min. and then cooled. Then the sheet so primed was coated with the above polyvinyl chloride powder composition with Stajet (Trademark, electrostatic powder spray coating machine of Societ Anonyme de Machines Electrostatiques, France) at jet velocity of 100 g./min. for about 10 sec., and immediately heated in an air oven at 200i5 C. for 15 min., holding the coated surface vertical. It required 240 sec. for the polyvinyl chloride powder composition to reach its softening point 110 C.

particularly the deposition of the powder composition 20 during the heating process.

TABLE 2 Experiment number 7 8 9 10 11 12 Composition of primer:

Polyvinyl chloride so 8 M R 80 N0 primer used. Phenol resin- 20 20 20 20 20 Graphite powder 0 5 20 5O 100 Volume resistivity (ohm.-cm.) 10" 5X10- 6X10- 2X10" 1X10 Results of experiment:

Falling off during heating powder compositinn None. N None None Partly falling ofi- Partly falling ofi. Average thickness of moltenfilm (y) 50 9Q "23G 250 240 250. Uniformity of molten film Pin holes throughout- Pm holes Good Good was more diflicult on the thick portion of the dry primer EXAMPLE 3 film, thus failing to form a uniform film and cissings were observed throughout the coated film. When the volume resistivity of the dry film of the primer was controlled to within 10 to 10 ohm-cm., excellent polyamide molten film free of the above faults was obtained.

EXAMPLE 2 400 parts of methylisobutylketone solution of copolymer of vinyl chloride, vinyl acetate and maleic anhydride, containing 20 percent non-volatile material and 40 parts of methylisobutylketone solution of resol type phenol resin containing 50 percent non-volatile material were mixed, to which was added graphite particles in the proportions as shown in Table 2 below and passed through 3-roll mill for three times to mix the components homogeneously, whereby primer composition was obtained.

The powder composition was prepared by mixing 100 parts of polyvinyl chloride particles, parts of dioctylphthalate, 3 parts of organic tin compound and 6 parts of rutile type titanium dioxide, and stirring the mixture with a Henshell mixer at 3000 rpm. for 30 min. The resultant polyvinyl chloride powder composition had the following characteristics:

Falling 06 during heating of Falling ofl None None.

powder composition.

Experiment number Average thickness of molten 250 230.

film (p).

Uniformity of molten film Good..-.. Good.

EXAMPLE 4 The primer of Example 1 was coated on a cold-rolled steel plate in the same manner as in Example 1 except varying the thickness of the dry primer film as shown in Table 4 below, and the polyvinyl chloride powder com- Grain size All passing mesh. position of Example 2 was applied thereto in the same Softening point C. manner as in Example 2 with the results shown in Table Volume resistivity 10 ohm-cm. 4 below.

TABLE 4 Experiment number 16 17 18 19 20 21 Composition of primer:

Phenol resin (parts). 100 100 100 100 100 100. Aluminium powder (parts) 5... 5 5. 20 20 20. Volume resistivity of dry primer film (ohm-cm.) 2X10" 2X10" 2X10 1X10- 1X10 1X10".

Thickness of dry primer film (p)-.. 25 48 4 28 42. Results of experiment:

Falling off during heating of deposited powder.. None N0ne-.--. None None".-- None".-. None. Average thickness of moltenfilm (u) 90 Almost no deposition.-- 220 220 Uniformity of molten film Pin holes. Pin holes throughout Good-.- Good- Good.

A cold-rolled steel sheet, 200 mm. x 100 mm. x 5 mm. which was subjected to zinc phosphate parkerizing according to Bonderite 144 standard process was coated with the primer at Table 2 diluted with methylisobutylketone to obtain dry film about 5 1. thick by spraying method, and after being left at room temperature for 5 min. was heated in a hot-air drier at l80i5 C. for

What we claim is:

1. A method for dry electrostatic coating a base metal with resin powder which comprises the steps of (1) coating the surface of the base metal with the primer of a vehicle and a particulate electroconductive material dispersed in the vehicle, said vehicle being an organic solvent solution of at least one binder resin selected from the group consisting of phenolic resin, epoxy resin and vinyl chlorideresin, said solution having a concentration of to weight percent based on the combined weight of the resin and organic solvent, said electroconductive material being one species selected from aluminium powder, silver powder, copper powder, bronze powder, acetylene, black powder and graphite powder, and having a particle size of not more than 100 1, and being contained in the amount ranging from 10 to weight parts based on weight parts of the binder resin in the vehicle, and being necessary for producing a dry film having a volume resistivity of 10 to 10 ohm-cm,

(2) heating the primer film at :5" C. for 10 to 20 minutes to obtain the dry film having a thickness of at least 3,u. and a volume resistivity of 10 to 10 ohm-cm.,

(3) electrostatically coating the resin powder on the primer film thus obtained, and then,

(4) heating the resin powder to melt and form a continuous film on the primer film.

2. The method according to claim 1, in which said particulate electro-conductive material has a particle size ranging from 0.01 to 101.4.

3. The method according to claim 1, in which said particulate electro-conductive material is contained in the range of 20 to 50 weight parts, based on 100 weight parts of the binder resin.

4. The method according to claim 1, in which said particulate electro-conductive material is contained in the amount necessary for producing a dry film having a volume resistivity of 10 to 10 ohm-cm.

5. The method according to claim 1, in which said primer is coated on the base metal in a dry film thickness of 10 to 100 .1.

6. The method according to claim 1 in which said resin powder is at least one species selected from epoxy resins, olefin polymer, polyesters, polyamides, vinylchloride polymers, polycarbonates or cellulose derivatives.

7. The method according to claim 6 in which said resin powder contains pigments and fillers.

References Cited UNITED STATES PATENTS 3,497,376 2/1970 Wieser 117--17 3,672,927 6/1972 Spiller et al 117-17 3,502,492 3/1970 Spiller 117--l7 3,327,685 6/1967 Heyl et a1 117-33 3,056,750 10/1962 Pass 252512 3,083,169 3/1963 Weda 2525l2 3,278,455 10/1966 Feather 252512 3,328,317 6/1967 McKeand et al 2525l1 3,332,867 7/1967 Miller et a1. 252-512 3,496,911 2/ 1970 Chmelar 11717 3,551,178 12/1970 Chmelar 11717 3,030,237 4/1962 Price 117-227 3,412,043 11/1968 Gilliland 117-227 3,577,274 5/1971 Taylor et al. 117227 3,639,121 2/1972 York 117--226 OTHER REFERENCES MICHAEL SOFOCLEOUS, Primary Examiner US. Cl. X.R. l17226, 227 

